This invention is concerned with a method by which a large number of identical spots can be generated at great speed from a laser beam. In the method of a laser beam is deflected through perpendicular planes by means of non-mechanical laser beam deflectors, and the deflection is transformed into a one-dimensional deflection using suitable optics which contains lenses, holograms, mirrors and prisms.
Previously a number of different one-dimensional laser beam scanning methods for making spots are known. These methods contain both mechanical and non-mechanical components. A summary of the most important scanning methods is presented in an article by Leo Beiser "Laser scanning and recording: developments and trends", Laser Focus/Electro-Optics (February 1985).
In mechanical scanning methods the laser beam is deflected by a moving galvanometer mirror or a rotating polygon, and the laser beam is focused using various lenses and holograms, which are located before or after the deflecting element. A method in which moving holograms are used has also been presented. The beam may be focused by these holograms or the focusing is effected by some other elements.
In non-mechanical deflectors the beam deflection is accomplished by acousto-optic or electro-optic laser beam deflectors and the beam focusing is due to some external optical elements. A method has also been presented in which focusing is accomplished by a non-mechanical deflector (chirp-deflector, U.S. pat. No. 3,851,951).
The disadvantages of mechanical deflectors are their complicated construction and very high costs when high resolution, about 10,000 spots, is required, because the necessary mechanics has to be very precise, and the focusing optics is very complex and expensive.
The disadvantage of non-mechanical deflectors is their quite low resolution (about 1000). With the so-called chirp-deflectors one can obtain a resolution of about 10,000 but these deflectors are extremely complex and very expensive.
A non-mechanical laser beam deflector has been presented in the patent U.S. Pat. No. 4,307,929 (Eveleth). In the method presented in that patent the laser beam is deflected by acousto-optic laser been deflectors to two mutually perpendicular directions. The signal to the second deflector is quite complex, which causes the presence of many different acoustic fields simultaneously in the deflector. Moreover, each spot is created by a separate hologram. The method presented in the present patent application differs from that method both in principle and realization. In our method a number of spots are formed by one hologram or grating, and the signals in the non-mechanical deflectors are quite simple. Thus, the method is quite simple because the number of holograms and gratings is low and the resolution required of the non-mechanical deflectors is also quite low.
A characteristic of the method presented here is also the fact that no mechanically moving parts are used in it, and that especially the holograms are not moving. Previously, there has been a patent on a laser scanning method which uses non-moving holograms (U.S. pat. No. 4,073,566, Noguchi), but this method uses a rotating polygon as the laser beam deflecting element. In the patents U.S. Pat. No. 4,547,037 (Case) and U.S. Pat. No. 4,106,844 (Bryngdahl et.al.) holograms are used to create the image spots. However, the separate spots are obtained by moving the holograms, in contrast to the present method.
Part of the invention concerns a method to eliminate the placement error of the spots. In many earlier patents there have been discussions on eliminating the placement error perpendicular to the line formed by the spots, but the present method greatly differs from these earlier patents. Firstly, the method presented here is non-mechanical, in contrast to many other patents. Secondly, our method has the characteristics that in the direction in which the error is to be eliminated, it can be done at all the deflection angle values of the non-mechanical deflector, in contrast to the other patents, in which one wants to keep the direction consistant.
In the known patents and constructions, a high resolution, inexpensive non-mechanical laser beam deflector has not been presented. The limiting factors have been the inadequate resolution of non-mechanical deflectors or the unreasonably complex structure of the systems.